A high-frequency filter is a filter the operating frequency of which falls in the range of a few hundred megahertz to tens of gigahertz. Known high-frequency filters are based on a few known basic resonator types. Resonator types include the helix resonator, in which the inner conductor is a cylindrical coil conductor and the outer conductor an electrically conductive casing; the dielectric resonator, in which the inner conductor is the electrically conductive coating of a hole formed in the dielectric body block and the outer conductor is the coating on the outer surface of that same body block; and the coaxial resonator, in which the inner conductor is a straight conductor the cross section of which is constant or changes in a step-like manner at some points, and the outer conductor is an electrically conductive casing not unlike the one in the helix resonator. The filter types are called helix filters, dielectric filters or coaxial filters according to the resonator type used. The present invention is applicable to filters in which the resonators' outer conductor is the electrically conductive casing. Resonators are also called filter circuits, and each portion of the casing that includes the inner conductor of a resonator is called a compartment.
In known high-frequency filters, the desired zeroes in the filter's frequency response are realized by means of link chains. To adjust the coupling, specially designed coupling elements are needed. The couplings required for achieving the desired response may be capacitive or inductive in nature or they may have both capacitive and inductive characteristics. A disadvantage of the link chain is that when a portion of the chain is adjusted, the adjustment tends to have a considerable effect on inter-circuit couplings as well as on the frequency, bandwidth and other characteristics of the circuits. Therefore, it is very difficult to set the specifications for the link chain. Because of the nature of the coupling and the construction of the filter the electrical implementation and mechanical construction of the coupling elements are unique in each individual case and therefore the coupling elements are expensive and laborious to manufacture. In addition, the assembly process is hard to automate.
U.S. Pat. No. 5,608,363 discloses different coupling methods in a filter construction which comprises at least two parallel resonator rows. The frequency response zeroes in the filter are realized by means of coupling elements attached to the walls between the resonator cavities. Disadvantages of the coupling elements described in the patent include the need for an economical and reliable attachment method for the necessary insulating material and the coupling element proper.